packPNM is a compression tool specifically designed for lossless compression of PPM, PGM, PBM and BMP. As of version 1.6 it also includes experimental Radiance HDR support (enable it with ‘-p’). Typical results vary with the content of the image compressed.

Download it from here:
packPNM v1.6d (LGPL)

packPNM has been in development almost as long as packJPG. There are no known bugs and it can be considered stable software (still, at least verify your files before deleting any originals!).